The invention relates to a method of setting the current intensity of an electron beam in a pickup tube of a television camera. In such a camera, the electron beam scans a target plate electrode in the pickup tube.
A television camera system employing such a method is described in Netherlands Patent Application No. 8,104,143 (corresponding to U.S. Pat. No. 4,331,980). In the television camera, an image of a scene being recorded is formed on the target plate electrode. The image generates a potential pattern on the target which corresponds to the brightness of the image of the scene. Brighter scene portions generate higher potentials on the target than dimmer scene portions.
Portions of the target having a higher potential than a nominal potential can be stabilized by increasing the beam current in one single field scan. For example, increasing the beam current intensity above the nominal value, such as to twice the nominal value, increases (doubles) the charge transfer to the target plate electrode. If, however, the maximum possible charge transer is not sufficient, stabilization in one single field scan cannot occur. On display of the picture signal, this results in comet tails behind moving, very bright scene portions. The stabilization then requires more than one single field scan.
In order to increase the beam current sufficiently to eliminate comet tails, but not excessively to cause defocussing, it is known, to increase a camera lens aperture by means of a diaphragm by a factor of two times the nominal opening. The luminous flux is then increased also by a factor of two, and the beam current is set such that on display of the generated picture signal no comet tails appear behind very bright scene portions.
It is an object of Netherlands Patent Application 8,104,143 to provide a beam current setting method in which no changes are made in the optical path of the camera, as such changes might give rise to several diasadvantages and problems. To that end, each field scan of the pickup tube target is made at a variable rate. More specifically, the target is scanned alternately at half the nominal rate, twice the nominal rate and half the nominal rate, each over one-third of the field scan. As a result, in the central third of the field the doubled scan rate halves the possible charge transfer via the electron beam or halves the effective beam current intensity, so that at very bright scene portions the picture is disturbed. The nominal beam current is now increased until there are no distorted scene portions any longer in the central third part of the picture. Therefore the normal field scan follows.